Plain English Meaning
A straight, clear path between two points with nothing solid in the way. For radios that work this way, if a mountain or the curve of the earth is between you and the station, the signal can't get through.
Full Definition
A straight, unobstructed path between a transmitter and a receiver. Radio signals at VHF and higher frequencies (including DME) travel in essentially straight lines and cannot bend around the curvature of the earth or pass reliably through terrain, so the aircraft and the ground station must have a direct line between them for the signal to be usable.
Context Anchor
Seen in discussions of Distance Measuring Equipment (DME), navigation radio reception, and why some signals become unreliable at low or behind terrain.